This invention relates to optical ring resonators. More particularly this invention relates to interferometric ring lasers with multiple ring cavities that are optically coupled together.
Semiconductor diode lasers with a ring resonator are an alternative to linear-cavity (Fabry-Perot) lasers for photonic integrated circuits and other applications. A ring resonator utilizes total internal reflection in a circular or other shaped ring structure, eliminating the need for end-facet mirrors and simplifying integration with waveguides, detectors, and other photonic integrated circuit components.
Current diode laser sources for photonic integrated circuits are primarily of the distributed feedback and distributed Bragg reflector types, requiring complicated device processing (etching of submicron diffraction gratings followed by regrowth of semiconductor cladding layers). Alternative diode laser sources (e.g. cleaved-coupled-cavity lasers) cannot be coupled to photonic integrated circuits without air-gap coupling which causes interference effects and undesirable feedback.
A new type of diode laser source, the ring resonator laser disclosed herein, is well-suited to photonic integrated circuits. This laser uses total internal reflection in a ring structure without end-facet mirrors so that it can be direct-coupled to waveguides and other photonic integrated circuit components. Previously, we have designed and fabricated single-quantum-well circular branching-waveguide Y-junction ring lasers to operate CW (J. P. Hohimer, D. C. Craft, G. R. Hadley, G. A. Vawter, and M. E. Warren, "Single-Frequency Continuous-Wave Operation of Ring Resonator Diode Lasers", Appl. Phys. Lett. 59, 3360 (1991)). These lasers were the first circular rib-waveguide Y-junction ring lasers to operate in a continuous wave (CW) mode. We also reported the first measurements of the size dependence of the lasing threshold current density in these devices (J. P. Hohimer, D. C. Craft, G. R. Hadley, and G. A. Vawter, "CW Room-Temperature Operation of Y-Junction Semiconductor Ring Lasers", Electron. Lett. 28, 374 (1992)). Our lowest CW threshold current for these devices was 40 mA (530 A/cm.sup.2), with single-frequency operation (&gt;20 dB side-mode suppression) and about 1 mW of TE-polarized output. We were also the first to report the deleterious effect that the outcoupling Y-junction can have in limiting the output power in these lasers by controlling the lasing ring modes.